Therapies directed at HER2 in breast cancer establish a successful paradigm that suggests other molecular-targeted treatments may prove useful in this disease. Though their functions overlap, laboratory data suggest that HER2/HER3 heterodimers predominantly activate the Akt/PI3-kinase cell survival pathway, while HER1/HER2 heterodimers mainly activate cell proliferation by the ERK1,2 MAP-kinase pathway. Trastuzumab, a humanized monoclonal antibody against HER2, is highly efficacious in breast cancer. Our preliminary data in human breast cancer biopsies obtained from a neoadjuvant trastuzumab clinical study indicates that the main mechanism of action of trastuzumab is by affecting Akt/PI3-kinase survival pathways, thereby inducing apoptosis, without significant changes in cell proliferation (Ki67 and p27). We and others have laboratory data suggesting that therapies designed to block both HER1/HER2 and HER2/HER3 pathways might be superior to either strategy alone in human breast cancers. We hypothesize that a small molecule like GW572016 which inhibits both Akt/PI3-kinase cell survival and ERK 1,2 MAP-kinase cell proliferation pathways by blocking HER1 and HER2. thereby inducing apoptosis and decreasing cell proliferation as dual mechanisms of action, will be an effective single agent in HER1/HER2 over-expressing breast cancer, and may be superior to therapies that affect either pathway alone (1). To test this hypothesis, we propose to perform a neoadjuvant clinical trial with GW572016, in which serial cancer tissue samples will be obtained for molecular studies in relation to tumor response. The following specific aims are proposed: (1) To demonstrate the clinical efficacy of GW572016 in patients with HER1/HER2-overexpressing locally advanced breast cancer, with and without gross concomitant metastatic disease, by assessing in a neoadjuvant trial the clinical response rate and toxicity of GW572016 in treatment-naive patients, and to obtain serial specimens from primary breast cancers at different time-points. (2) To determine if GW572016 inhibits HER1 and HER2 signaling in vivo in these sequential core biopsies by assessing cell survival pathways (apoptosis by cleaved caspase 3, and phosphorylated Akt), cell cycle arrest (ERK1,2 MAP-kinase, Ki67, and p27), and down-regulation of total and phosphorylated HER1 and HER2. (3) To identify predictive markers for sensitivity and resistance to GW572016 by gene expression array analysis of pretreatment samples. With normal vasculature and surrounding stromal milieu, these serial samples of human breast cancer will provide important information on the in vivo mechanisms of action of GW572016, a promising novel small molecule with dual specificity against HER1 and HER2, together with molecular signatures that may predict response and resistance to this agent.